You know, I forgot the first thing that tech-support people tell you (after they ask you to make sure your computer is plugged in and turned on) -- Reboot and see if that helps.

Lo and behold. That cleared up the lag I was experiencing. There must have been some bugginess that had accumulated, that SR1 can't handle as well as the original release. I feel sort of foolish for not trying that before raising the issue. And yes, my Mac was plugged in!